The metabolism and excretion of buprenorphine in humans

Buprenorphine, a powerful mixed agonist-antagonist analgesic which shows promise of providing maintenance pharmacotherapy for heroin addicts, is metabolized in male human subjects to norbuprenorphine and to conjugated buprenorphine and norbuprenorphine. Following subcutaneous, sublingual, and oral buprenorphine administration to a single subject, total metabolite excretion in urine was 2, 13.4, and 12.1%, respectively. No free parent drug was detected in urine. The amount of norbuprenorphine metabolite excreted in urine generally exceeded that of conjugated buprenorphine. In contrast, free and conjugated buprenorphine equaled or greatly exceeded total norbuprenorphine content in fecal samples following oral or sublingual administration. The greatest amount of drug and metabolite eliminated in feces occurred at 4 to 6 days following buprenorphine administration at times when there was very little urinary excretion of conjugated buprenorphine. This latter evidence indicates an enterohepatic circulation of buprenorphine in humans.